Transfer RNA

Translation of the codons is accomplished by tRNA and particular enzymes. Each tRNA molecule, like mRNA and rRNA, is single-stranded. Although tRNA is single-stranded, it bends in on itself to form a cloverleaf structure (fig. 3.22a), which is believed to be further twisted into an upside down "L" shape (fig. 3.22b). One end of the "L" contains the anticodon— three nucleotides that are complementary to a specific codon in mRNA.

Enzymes in the cell cytoplasm called aminoacyl-tRNA synthetase enzymes join specific amino acids to the ends of tRNA, so that a tRNA with a given anticodon can bind to only one specific amino acid. There are twenty different varieties of synthetase enzymes, one for each type of amino acid. Not only must each synthetase recognize its specific amino acid, it also must be able to attach this amino acid to the particular tRNA that has the correct anticodon for that amino acid. The cytoplasm of a cell thus contains tRNA molecules that are each bonded to a specific amino acid, and each of these tRNA molecules is capable of bonding with a specific codon in mRNA via its anticodon base triplet.

Table 3.2 Selected DNA Base Triplets and mRNA Codons

DNA Triplet RNA Codon Amino Acid

TAC

AUG

"Start" (Methionine)

ATC

UAG

"Stop"

AAA

UUU

Phenylalanine

AGG

UCC

Serine

ACA

UGU

Cysteine

GGG

CCC

Proline

GAA

CUU

Leucine

GCT

CGA

Arginine

TTT

AAA

Lysine

TGC

ACG

Threonine

CCG

GGC

Glycine

CTC

GAG

Glutamic acid

DNA coding strand

T

A

C

C

C

G

A

G

G

T

A

G

C

C

G

C

G

T

C

G

T

J

■J-

"V

"V

r

J~

J~

r

r

A

U

G

G

G

C

U

C

C

A

U

C

G

G

C

G

A

G

C

Messenger RNA

Codon 1 Codon 2 Codon 3 Codon 4 Codon 5 Codon 6 Codon 7

Codon 1 Codon 2 Codon 3 Codon 4 Codon 5 Codon 6 Codon 7

■ Figure 3.21 Transcription and translation. The genetic code is first transcribed into base triplets (codons) in mRNA and then translated into a specific sequence of amino acids in a polypeptide.

Cell Structure and Genetic Control

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